I'm interested in whether or not you would limit the redefinition of marriage to couples or extend it to any number of people, be it a sexual or platonic relationship.
If you would extend it, then that is a principled position though one I would take a dim view of.
You're also conflating issues as regards marriage. The fact that there are people who do not practice marriage as it should be practiced says little to nothing about the institution of marriage. Similarly, the fact that marxists abound in our Constitutional Republic says nothing about the worth of our Constitution.